The present invention in general relates to a microwave voltage controlled oscillator, such as the automobile radar, that performs narrow band modulation. More particularly, this invention relates to a microwave voltage controlled oscillator in which frequency modulation linearity is improved.
A conventional microwave voltage controlled oscillator will be described. FIG. 1 is a diagram showing a principle configuration of a typical microwave voltage controlled oscillator. Legend 61 denotes a negative resistance element (circuit), legend 62 denotes a resonator circuit, and legend 63 denotes a variable capacitance diode. This microwave voltage controlled oscillator is a well known combination of a variable capacitance diode and negative resistance.
It will be assumed here that, the impedance of the resonator circuit 62 which is to the left of the reference line A-Axe2x80x2 is ZR, and that the impedance of the negative resistance element 61 which is to the right of the reference line A-Axe2x80x2 is ZN. With this assumption in mind, the microwave voltage controlled oscillator oscillates stably when the following equation (1) holds true. That is,
Re{ZR+ZN} less than 0
Im{ZR+ZN}=0xe2x80x83xe2x80x83(1)
d{ZR+ZN}/dxcfx89 greater than 0
where xcfx89 represents angular frequency.
It will be further assumed that the impedance of the variable capacitance diode 63 (capacitance CV) which is to the left of the reference line B-Bxe2x80x2 is 1/jxcfx89CV, and that the impedance of the resonator circuit 62 which is to the right of the reference line is RT+jxcfx89LT. With this assumption in mind, the oscillation frequency fosc of the oscillator can be represented by the following equation (2).
fosc=1/[2xcfx80(LTCV)1/2]xe2x80x83xe2x80x83(2)
When the variable range of the frequency is very narrow and LT can be regarded as constant in the whole variable frequency range, an oscillation frequency proportionate to the control voltage can be obtained if the capacitance CV is in inverse proportion to the square of the control voltage.
However, a variable capacitance diode having a capacitance CV which is in inverse proportion to the square of the control voltage is not commercially available. Furthermore, as for a special diode capable of operating at a frequency equivalent to at least a microwave frequency, it is difficult to obtain even if it exists. Under a condition that LT is constant, therefore, a linear voltage controlled oscillator as described above has not been implemented.
FIG. 2 is a diagram showing a circuit configuration of the conventional microwave voltage controlled oscillator. Legend 64 denotes the above described shorted quarterxe2x80x94wavelength line as high impedance shunt stub. The quarter wavelength line becomes infinite in impedance at an oscillation frequency, and it does not affect the oscillation operation at the microwave frequencies. Therefore, the voltage-capacitance characteristic of the variable capacitance diode determines the linearity of frequency modulation.
FIG. 3 is a diagram showing a variable frequency characteristic and frequency modulation linearity (% indication) of the conventional microwave voltage controlled oscillator shown in FIG. 2. Specifically, FIG. 3A represents the oscillation frequencyxe2x80x94control voltage characteristic, and FIG. 3B represents the linearity of frequency modulation.
The linearity (i.e. the linearity index) of frequency modulation is defined by the oscillation frequencyxe2x80x94control voltage characteristic shown in FIG. 3A. For example, assuming that a maximum deviation between a straight line coupling both ends of a frequency modulation width W and illustrated frequencies is xcex94W, the linearity index can be represented by 100xc3x97xcex94W/W (%). To be concrete, the linearity index becomes at least 20% as shown in FIG. 3B.
This does not exert influence upon fixed frequency oscillators for communication. For example, in devices for transmitting and receiving a frequency modulated signal and measuring a distance between cars, such as a crash prevention radar, however, this indicates a value remarkably degrading the precision of measurement results. In other words, even in the case where it is attempted to improve the precision by using some external frequency linearizer such as using ROM, the linearity index can not be suppressed enough. In the case of crash prevention radar, however, the frequency modulation width is negligibly small as compared with the oscillation frequency and a nearly fixed frequency can be considered.
A conventional linear microwave voltage controlled oscillator will now be described in detail by referring to a concrete example. FIG. 4 is a diagram showing a configuration of a microwave voltage controlled oscillator (high frequency device) described in Japanese Patent Application Laid-Open No. 8-288715. Legend 31 denotes a resonator circuit, legend 32 denotes a capacitance (realized with a capacitor), and legend 33 denotes a negative resistance circuit (realized with amplifier circuit).
Furthermore, in the resonator circuit 31, legend 47 denotes a main resonance line, legend 48 denotes a DC-cut capacitance, and legend 49 denotes a variable capacitance diode having a capacitance which is in inverse proportion to the square of voltage. Legend 52 denotes a series connection of a strip line 50 for correcting the square characteristic of the variable capacitance diode 49 to obtain a linear characteristic and a capacitor 51. Legend 53 denotes a strip line for further improving the correction effect of the strip line 50. Legend 54 denotes a terminal, and legend 55 denotes a capacitor. In the negative resistance circuit 33, legend 34 denotes a transistor, legends 35, 36, 37 and 38 denote resistors, legends 39, 40, 41, 42 and 43 denote capacitors, legend 44 denotes a coil, and legends 45 and 46 denote terminals.
In this microwave voltage controlled oscillator, for example, when a strip line is used as a distributed transmission line in the resonator circuit 31 and the variable frequency range is wide, LT changes linearly with the frequency. Even when the capacitance CV is inverse proportion to the square of the voltage, the oscillation frequency is not proportionate to the control voltage. Therefore, a correction circuit (corresponding to the strip line 50 and the strip line 53) is incorporated so that the capacitance of the variable capacitance diode 49 will change linearly with the applied voltage. Furthermore, lengths of the strip lines 50 and 53 are defined as {fraction (1/10)} wavelength to {fraction (1/20)} wavelength and {fraction (3/16)} wavelength to {fraction (5/16)} wavelength, respectively, in order to modulate the oscillation frequency in a wide frequency band.
An explanation will be given about the strip line 50. If the strip line 50 is not provided, changing the voltage only slightly causes an abrupt change of the above described variable capacitance when the control voltage is low, resulting in an abrupt change of the oscillation frequency. When the control voltage is high, the oscillation frequency changes gently as the voltage changes.
However, when strip line 50 is provided, the strip line 50 changes the impedance abruptly, and consequently the oscillation frequency changes gradually. In keeping therewith, the oscillation frequency changes linearly as the control voltage increases. In order to satisfy the expression xe2x80x9cthe strip line 50 changes the impedance abruptly, and consequently,xe2x80x9d it is important that the impedance of the correction circuit has a great change at different frequencies. Especially here, the series connection 52 for generating a great impedance change with respect to a frequency is connected in parallel with the variable capacitance diode 49.
In the conventional microwave voltage controlled oscillator, the correction circuit (the strip line 50 and the strip line 53) is thus incorporated so that the capacitance of the variable capacitance circuit with diode 49 may change linearly with the applied voltage. Furthermore, by determining the lengths of the strip lines 50 and 53 to be {fraction (1/10)} wavelength to {fraction (1/20)} wavelength and {fraction (3/16)} wavelength to {fraction (5/16)} wavelength, respectively, the oscillation frequency is modulated linearly in a broad frequency band.
In a crash prevention radar, however, frequency modulation of approximately 50 MHz is conducted in a 38 GHz band, for example, a frequency modulated wave of a 76 GHz band obtained by frequency multiplication thereof with a ratio of two is transmitted and received, and a distance between the radar and the target is measured. In this case, a great difference from a fixed frequency disappears. Therefore, a frequency change width for abruptly changing the impedance does not exist. To be concrete, there is a problem that the frequency band becomes very narrow with the specific bandwidth equivalent to approximately 0.3% and the function of the above described correction circuit cannot be used.
Furthermore, in the conventional microwave voltage controlled oscillator in which a correction value CM of capacitance CV of the variable capacitance diode is changed linearly with the voltage in operation close to the above described fixed frequency, LT is a nearly fixed value as evident from the equation (2) (CV in the equation (2) is replaced with CM). Since LT is a nearly fixed value, there is obtained such a variable frequency characteristic that the oscillation frequency fosc becomes proportionate to (xe2x88x92xc2xd)th power of the voltage. In this case, the modulation linearity value becomes at least 20%. (In the change rate of a modulation sensitivity in a constant modulation width, the dependence of the oscillation frequency upon the voltage approaches a straight line as the modulation linearity value approaches 0%.) This results in a problem that the conventional microwave voltage controlled oscillator becomes unsuitable for the voltage controlled oscillator for crash prevention radar.
It is an object of the present invention to obtain a microwave voltage controlled oscillator which can be applied to a crash preventing radar by improving the modulation linearity value as compared with the conventional technique.
In the microwave voltage controlled oscillator according to this invention a negative resistance circuit having an external output terminal is connected to a first terminal of a strip shaped resonator, and an anode of a variable capacitance diode is connected to a second terminal of the strip shaped resonator via first capacitance, a cathode of the variable capacitance diode is connected to ground, a first terminal of a high impedance strip shaped line is connected to the anode of the variable capacitance diode, and a second terminal of the strip shaped line is connected to the ground via second capacitance having sufficiently low impedance at an oscillation frequency. Furthermore, length of the strip shaped line is set to a value in a range of xc2xc wavelength to {fraction (1/14)} wavelength.
Because of such a configuration, and because the line length of the strip shaped line is in the range of xc2xc wavelength to {fraction (1/14)} wavelength, the linearity of the frequency modulation can be remarkably improved as compared with, for example, when frequency control is effected by using only a diode.
Furthermore, by utilizing the fact that negative admittance equivalently obtained from the strip shaped line is connected to the variable capacitance diode, the length of the strip shaped line is determined so that sum of capacitance of the variable capacitance diode and the admittance will be in inverse proportion to the control voltage. By this determination, the conventional linearity index can be remarkably improved. Furthermore, since the remarkable improvement of the linearity index can facilitate the application of the linearizer, an oscillator for crash preventing radar can be effectively implemented.
Furthermore, by utilizing the fact that negative admittance equivalently obtained from the strip shaped line is connected to the variable capacitance diode, the length of the strip shaped line is determined so that sum of capacitance of the variable capacitance diode and the admittance will be in inverse proportion to first to second power of the control voltage. As compared with a microwave voltage controlled oscillator using a variable capacitance diode which is in inverse proportion to the one-half power of the control voltage, therefore, a twice line length can be used. Furthermore, it is possible to prevent lowering in design efficiency and reproducibility resulting from the line length of the strip shaped line becoming too short.
The length of the strip shaped line is fixed to xc2xc wavelength, and the strip shaped line is connected to the ground in a predetermined position depending on a characteristic of the variable capacitance diode in use. By fixing the line length of the strip shaped line to xc2xc wavelength and connecting the strip shaped line to the ground in an arbitrary position, therefore, fine adjustment can be performed so as to absorb dispersion of the CV characteristic of the variable capacitance diode without limiting the kind of the variable capacitance diode. As a result, the linearity of the frequency modulation can be further remarkably improved.
Furthermore, a Schottky diode or a FET diode is used as the variable capacitance diode. When the Schottky diode or the FET diode is used, the linearity of the frequency modulation can also be remarkably improved as compared with the conventional technique.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.